1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a hetero-junction field effect transistor which operates at a high speed with a low noise and a microwave monolithic integrated circuit (MMIC) which uses the FET.
2. Related Background Art
A hetero-junction field effect transistor (or high electron mobility transistor (HEMT)) which uses a selectively doped hetero-junction has been proposed as a high operating speed transistor. FIG. 1 shows a structure of a typical AlGaAs/GaAs high electron mobility transistor. The structure is described below.
An undoped GaAs layer 210 is formed on a semi-insulating GaAs substrate 110, and an ALGaAs layer (undoped AlGaAs spacer layer) 220 having a smaller affinity than that of GaAs is formed on the GaAs layer 210. The AlGaAs layer consists of the undoped AlGaAs spacer layer 220 and an AlGaAs layer 230 doped with n-type dopant (an element such as Si or Se), which are formed on the GaAs 210. A gate electrode 250 is formed on the AlGaAs layer, and a source electrode 270 and a drain electrode 280 are formed on a Si-GaAs contact layer 260 to sandwich the gate electrode 250. This electrode structure is called a recessed structure because the gate electrode is provided at the bottom of the grove (recess), and it is a common gate electrode structure in the HEMT. By this structure, two-dimensional electron gas 240 is formed on the side of GaAs on the interface of AlGaAs/GaAs, and it serves as a drain-source channel (current path). A density of the two-dimensional electron gas 240 is controlled by the gate electrode 250 and a current between the source electrode 270 and the drain electrode 280 is modulated.
On the other hand, a pulse doped MESFET has been reported in the paper ED89-152 of the Study Group of the Electronic and Information as a transistor other than the HEMT, which operates at a high speed with a low noise. The pulse doped MESFET uses a GaAs layer having Si pulsively doped as a channel and has a structure as shown in FIG. 2. The structure is described below.
An undoped GaAs buffer layer 310 having a p carrier conductivity type (carrier density .about.5.times.10.sup.15 cm.sup.-3) is formed on a semi-insulating GaAs substrate 110, and Si-doped GaAs (.about.14.times.10.sup.18 cm.sup.-3) 320 is formed with a thickness of 100 angstrom. An undoped GaAs cap layer 330 having an n carrier conductivity type (.about.1.times.10.sup.15 cm.sup.-3) is formed on the Si-doped GaAs channel layer 320. A profile of inpurity distribution is low in the GaAs buffer layer 310 and the GaAs cap layer 330, and pulsively high in the Si-doped GaAs layer 320. Therefore, this structure is called a pulse doped structure. A gate electrode 340, and n.sup.+ ion implantation layer 350a and 350b, a source electrode 360 and a drain electrode 370 which are self-aligned to the gate electrode 340 are formed on the pulse doped structure. This electrode structure is called a planar structure because the gate electrode is planar.
FIG. 3 shows examples of characteristics of the AlGaAs/GaAs HEMT and the pulse doped MESFET. It shows a gate bias dependency of a transfer conductance (gm) when a device having a gate length of 0.3 .mu.m is used. The pulse doped MESFET has a mesa profile (broken line) of the transfer conductance gm to the gate bias, and a change in the transfer conductance gm when a biasing point is slightly shifted is small, but the value of the transfer conductance gm is smaller than that of the HEMT. The abruptness of the rise of the transfer conductance gm at the gate bias near a threshold (Vth), which is important as a low noise device, is inferior to the HEMT.
On the other hand, the HEMT exhibits an abrupt rise of the transfer conductance gm and a peak thereof is high, but since it has a peak profile (chain line) to the gate bias, the transfer conductance gm is significantly reduced if the bias point is slightly shifted. The reduction of the transfer conductance gm of the HEMT in a shallow gate bias side is due to a phenomenon called a real space transition in which a portion of the two-dimensional electrons transists to the AlGaAs layer which has a low electron velocity.
FIG. 4 shows an energy band chart of the HEMT shown in FIG. 1 along a line X--X, in which Ec denotes a bottom of a conduction band and Ev denotes a Fermi level. As shown in FIG. 4A, as the gate voltage V.sub.GS is raised toward a positive side, a portion of the two-dimensional gas generated in the interface of the undoped GaAs layer 210 transits to the n.sup.+ AlGaAs layer 230 as shown in FIG. 4B. As result, a total electron mobility is reduced and the gm also is abruptly reduced from the peak.
The peak profile of the transfer conductance gm leads to a small design margin and a low yield for an integrated circuit (IC) when the integrated circuit is to be fabricated by using the HEMT. In the HEMT structure, since the abruptness in the interface of AlGaAs/GaAs is important, the planar gate electrode by the self-alignment ion implantation is not adopted. Because it is necessary to anneal the ion-implanted Si at a high temperature in order to activate it, and if Al in the AlGaAs layer is diffused into the GaAs layer in the annealing step, the electron mobility and a saturation velocity are significantly reduced. Accordingly, the gate electrode in the HEMT is usually of recessed type, and a variance of depth of the recess in an etching process to form the recess is reflected to a variance of Vth. From the aspect of the process margin, too, the prior art HEMT is not always suitable as a device for constructing an integrated circuit.